Generally, an animation with a video camera is conventionally used for recording the change over time (that is, the record of the change with passage of time regarding positions, shapes and colors of a certain object), but still images with a digital camera are used for recording the change fora longer time, such as several days and several months in some cases. In a field of medical treatment, for example, a degree of progression of condition and postoperative recovery degree are diagnosed by comparing two or more photographs irregularly obtained, such as fundus photographs. But, its comparison work is complex since two or more still images being in line should be compared with each other, and it is not easy to grasp the change with passage of time.
Then, as a method of solving such a problem, a method of composing a pseudo-video that shows the change of the object with passage of time with two or more still images has been proposed (see patent related document 1). Concretely speaking, it is possible to easily grasp the change of the object with passage of time in such a manner that two images (a former image and a later image) are superimposed, and the former image is gradually switched into the later image by gradually changing a transparency of the later image from 0% to 100%.